In software testing, a considerable attention is given to coverage. Coverage refers to the extent of which all parts of the tested code have actually been subjected to testing by the test tool.
To measure what percentage of the code has been executed by the test tool, coverage criterion or criteria are used, which essentially are requirement rules that need satisfying in order to verify coverage.
As tests are carried out repeatedly, and may involve long hours or days of repeated test runs, it is useful to determine the percentage of coverage has already been completed.